Many work machines, particularly earth working machines, use a continuously variable transmission to drive traction wheels or tracks of the work machine. Typically, a hydrostatic drive, consisting of a hydraulic pump and a hydraulic motor, provides a continuously variable speed output to the wheels or tracks of the work machine. In particular, the speed output can be continuously varied by controlling the displacements of either the hydraulic pump or the hydraulic motor which comprise the hydrostatic drive system.
In order to operate over a wide range of operating conditions, the output of the continuously variable hydrostatic transmission is transmitted through a mechanical transmission. The mechanical transmission has a number of discrete transmission ranges corresponding to different operating speeds of the work machine. The combination of the continuously variable transmission and the mechanical transmission allows a continuously variable transmission to operate over a wider range of speeds than is possible using the continuously variable transmission alone.
One drawback to having a number of transmission ranges is that power may not be smoothly transmitted through the transmission during a transmission range shift from one transmission range to another. Typically, an oncoming clutch takes a time to fill the volume of the clutch before actual engagement. Therefore, the acceleration of the work machine must be stopped to wait for the oncoming clutch to be filled and ramped up to pressure. This does not allow a constant and smooth acceleration through the shift. In particular, if a shift is performed before the desired transmission ratio, a shock may be transmitted through the transmission. On the other hand, if the shift is performed after the desired transmission ratio, the shift will appear slow. In either case, the operator has the perception that the transmission ratio is not being smoothly varied.
One of the advantages of the continuously variable transmission is that the continuous variability of the transmission ratio provides the operator with perception that the work machine is operating smoothly. If the shift from one transmission range to another is not performed at the desired transmission ratio, the perception of smooth operation is lost as the operator feels either the shock or the slow shift.
What is needed therefore is a method and apparatus for adaptively shifting ranges in a transmission which overcomes the above-mentioned drawbacks.